Known examples of brake apparatuses mounted on vehicles such as four-wheeled automobiles include disk brakes. According to the disk brakes, a hydraulic pressure is externally supplied into a cylinder of a caliper to push and move a piston toward a surface side of a disk together with a brake pad, thereby generating a braking force. Among these disk brakes, there are known disk brake apparatuses equipped with a parking brake function, which are configured to function as a parking brake when the vehicle is stopped or parked in addition to generating a braking force when the vehicle is running (PTLs 1, 2, and 3).